Universal Access: in particular, "Assistive Devices" – my first thought
was that I had inadvertently used a keyboard shortcut that triggered
this mode which is in fact an option under Assistive Devices

Deleted & Reinstalled iTunes: including the cache, temporary files in~/Library/Application Support and ~/Library/iTunes

I have also surfed the Apple Support forums, and indeed, there are several posts about this, but all of them unanswered.